Crabbé reaction

[1][2] Given continued developments in scope and generality, it is a convenient and increasingly important method for the preparation of allenes, a class of compounds often viewed as exotic and synthetically challenging to access.

The author noted that iron salts were completely ineffective, while cupric and cuprous chloride and bromide, as well as silver nitrate provided the desired product, but in lower yield under the standard conditions.

[4] Shengming Ma (麻生明) and coworkers at the Shanghai Institute of Organic Chemistry (SIOC, Chinese Academy of Sciences) investigated the reaction in detail, including clarifying the critical role of the base, and developed conditions that exhibited superior functional-group compatibility and generally resulted in higher yields of the allene.

In broad strokes, the mechanism of the reaction is believed to first involve a Mannich-like addition of the alkynylmetal species into the iminium ion formed by condensation of the aldehyde and the secondary amine.

In the second part, the α-amino alkyne then undergoes a formal retro-imino-ene reaction, an internal redox process, to deliver the desired allene and an imine as the oxidized byproduct of the secondary amine.